The present invention relates to a cholesteric liquid crystal layer suitable for use in, for example, improving the brightness and perceptibility of liquid crystal displays, etc. The present invention further relates to an optical element and a lighting device.
An optical element has conventionally been known which has a separation layer for circular light polarization comprising a cholesteric liquid crystal layer showing such circular dichroism that natural light is separated thereby into reflected light comprising circularly polarized light and transmitted light, and in which the circularly polarized light is converted to linearly polarized light with a quarter-wavelength plate. This element has an advantage that since the light emitted by a light source can be supplied as linearly polarized light to a polarizing plate, the absorption loss in the polarizing plate can be reduced to thereby improve the brightness of a liquid crystal display.
However, such conventional optical elements have a problem that the display appears to be colored when viewed from slant directions. It has been proposed to mitigate this problem concerning viewing angle by disposing a quarter-wavelength plate on the longer-helical-pitch side of a cholesteric liquid crystal layer having Grandjean orientation (see PCT Publication WO97/19385). However, this technique is unsuitable for practical use because the improvement in viewing angle characteristics is insufficient for application to liquid crystal displays and the like.
One object of the present invention is to provide a cholesteric liquid crystal layer which is inhibited from appearing to be colored when viewed from slant directions and thus has excellent viewing angle characteristics, while enabling an improvement in the brightness of liquid crystal displays, etc.
Another object of the present invention is to provide an optical element and a lighting device with each of which liquid crystal displays and the like excellent in brightness and perceptibility can be fabricated.
The present invention provides a cholesteric liquid crystal layer having Grandjean orientation in which the helical pitch changes in the thickness direction, wherein circular dichroism by which natural light is separated into reflected light comprising circularly polarized light and transmitted light is exhibited over a wavelength region having a width of at least 50 nm and including a wavelength of 520 nm, said cholesteric liquid crystal layer showing either such polarization characteristics that when the reflected light or the transmitted light is converted to linearly polarized light through a quarter-wavelength plate, then the linearly polarized light in the visible light region has a maximum value of the degree of polarization at a wavelength of 520 nm or shorter or such polarization characteristics that when the degree of polarization of the linearly polarized light in the visible light region is integrated in the wavelength region where the circular dichroism is exhibited, then the integrated degree of polarization for the region on the shorter-wavelength side of 520 nm is higher than that for the region on the longer-wavelength side of 520 nm.
The present invention further provides an optical element comprising a multilayer structure comprising the cholesteric liquid crystal layer described above and at least one optical layer superposed thereon, the optical layer being a retardation compensating plate, a polarizing plate, a light conductive plate, or a quarter-wavelength plate which comprises one or more retardation layers including a retardation layer having a value of Nz defined by the equation (nxxe2x88x92nz)/(nxxe2x88x92ny)=Nz of xe2x88x920.8 to xe2x88x922.0, wherein nx is the maximum in-plane refractive index, ny is the refractive index in a direction perpendicular to the nx direction, and nz is the thickness-direction refractive index.
The present invention furthermore provides a lighting device or liquid crystal display which comprises the cholesteric liquid crystal layer or optical element disposed over a surface light source or on one side of a liquid crystal cell.
This invention still further provides a liquid crystal display comprising the lighting device and a liquid crystal cell disposed on the light emission side thereof.
According to the present invention, a cholesteric liquid crystal layer, an optical element, and a lighting device can be obtained which each is inhibited from appearing to be colored when viewed from slant directions and thus has excellent viewing angle characteristics while enabling an improvement in the brightness of liquid crystal displays, etc. By use of these, a liquid crystal display or the like having excellent brightness and perceptibility can be fabricated. This is attributable to the above-described polarization characteristics of the cholesteric liquid crystal layer.
The present inventors made intensive studies in order to improve the viewing angle characteristics. As a result, it has been found that the colored appearance in viewing from slant directions can be inhibited and viewing angle characteristics are greatly improved not with any conventional cholesteric liquid crystal layer in which the linearly polarized light obtained through a quarter-wavelength plate has a constant degree of polarization throughout the visible wavelength region where circular dichroism is exhibited, but with a cholesteric liquid crystal layer in which the linearly polarized light has different degrees of polarization in the visible wavelength region, specifically, which shows such polarization characteristics that the linearly polarized light has a higher degree of polarization locally or wholly in a range of wavelengths of 520 nm or less, especially in a shorter-wavelength visible region of from 400 to 520 nm, than in a longer-wavelength visible region of especially from520to 700nm. The present invention has been completed based on this finding.